Many users of mobile electronic devices have a second battery on hand for use when the battery in the mobile electronic device is discharged. After discharging both batteries, a user will want to recharge the two batteries as quickly as possible.
Current options include the following:    a) Fully charging one battery using the charging circuitry in the mobile electronic device and an external power adapter, then exchanging the two batteries to charge the second battery.    b) Charging one battery using the charging circuitry in the mobile electronic device and an external power adapter, and simultaneously charging the second battery in an external battery charger;    c) Charging one battery using the charging circuitry in the mobile electronic device connected to a dual-output external battery charger, with the second battery connected to the other output of the external battery charger. The external battery charger charges the batteries in sequential order without user intervention, or trickle charges one of the batteries while charging the other battery at full rate until it is fully charged, or gives one battery priority and provides all available current to that battery and any remaining current that can be supplied to the charger is provided to the secondary battery.
These options either extend the charge time by charging the batteries in sequential order, by keeping one battery in slow-rate trickle charge until it is fully charged, or require the user to carry two separate charging accessories with them.